1. Field of the Invention
The present invention relates to the use of an array of DC accelerometers for detection of patient posture and activity level for medical monitoring and/or the delivery of therapies, including cardiac pacing.
2. Description of the Prior Art
In the field of medical device technology, patient monitoring of physiologic parameters e.g. heart rate, temperature, blood pressure and gases and the like are well known. In addition, the delivery of various therapies including drugs and electrical stimulation by implanted or invasive medical devices is well known. Factors that may be appropriately taken into account during monitoring or delivery of therapies include patient position or posture and activity level. Both may have an effect on the other parameters monitored and in the decision process for setting an appropriate therapy. Particularly in the field of cardiac pacing, patient activity level can be correlated to the need for cardiac output.
Rate responsive pacing has been widely adopted for adjusting pacing rate to the physiologic needs of the patient in relatively recent years. Early single chamber patient in relatively recent years. Early single chamber cardiac pacemakers provided a fixed rate stimulation pulse generator that could be reset, on demand, by sensed atrial or ventricular contractions recurring at a rate above the fixed rate. Later, dual chamber demand pacemakers became available for implantation in patients having an intact atrial sinus rate but no AV conduction, so that ventricular pacing could be synchronized with the atrial sinus rate, and backup fixed rate ventricular pacing could be provided on failure to sense atrial depolarizations. In addition, rate programmable pacemakers became available wherein the base pacing rate could be selected by a physician to provide a compromise fixed rate that did not interfere with patient rest and provided adequate cardiac output at moderate levels of exercise.
Such fixed rate pacing, particularly for patients not having an adequate atrial sinus rate to allow synchronous pacing, left most patients without the ability to exercise, lift objects or even walk up stairs without suffering loss of breath due to insufficient cardiac output. However, the introduction of the Medtronic.RTM. Activitrax.RTM. pacemaker provided patients with the a pulse generator having a rate responsive capability dependent on the level of patient activity. A piezoelectric crystal bonded to the interior of the implantable pulse generator can or case is employed in that pacemaker and successor models to provide a pulse output signal related to the pressure wave generated by a patient's footfall and conducted through the body to the crystal. Thus, low frequency activity signals recurring at the patient's rate of walking or running could be sensed and processed to derive a pacing rate appropriate to the level of activity. The activity sensor and its operation is described in commonly assigned U.S. Pat. No. 4,428,378 to Anderson.
Since the introduction of the Activitrax.RTM. pacemaker, a great many rate responsive pacemakers employing a wide variety of activity sensors and other physiologic sensors have been proposed and marketed. A comprehensive listing of such rate responsive pacemakers, sensors and sensed physiologic parameters is set forth in commonly assigned U.S. Pat. No. 5,226,413 to Bennett et al., incorporated herein by reference. However, the activity sensor of the type employed in the Activitrax.RTM. pacemaker continues to be used in successor single and dual chamber, rate responsive pacemaker models and remains the most widely used physiologic sensor.
As mentioned above, this piezoelectric crystal sensor is responsive to pressure waves generated by patient footfalls striking the exterior of the pulse generator case. Activity sensor configurations employing integrated circuit, AC accelerometers on an IC chip inside the pacemaker are also being employed in the EXCEL"VR pacemaker sold by Cardiac Pacemakers, Inc., and in similar rate responsive pacemakers sold by other manufacturers. The AC accelerometer is formed of a silicon beam mass suspended on the IC that swings or moves in response to shock waves caused by body motion and provides an output signal having a magnitude dependent on the rate of movement.
Like the piezoelectric crystal sensor, there is no signal output from the AC accelerometer in the absence of body motion and related to body position or attitude. In other words, when a patient is at rest, neither activity sensor provides any indication as to whether the patient is upright and awake and resting or lying down and presumably sleeping or resting. A lower sleep pacing rate than the rest pacing rate while awake and upright may be desirable for a given patient. Other sensors for sensing physiologic parameters induced by high levels of exercise have been proposed to detect the physiologic changes accompanying exercise, rest and sleep to trigger appropriate rates. Particularly, to lower the pacing rate during sleep, the inclusion of a real time clock to establish a Circadian rhythm pacing rate have also been proposed. None of these proposed sensors or systems are capable of determining a patient's position or posture.
A mechanical sensor has been proposed in the article "A New Mechanical Sensor for Detecting Body Activity and Posture, Suitable for Rate Responsive Pacing" by Alt et al. (PACE, Vol. 11, pp. 1875-81, November, 1988, Part II) and in Alt U.S. Pat. No. 4,846,195 that involves use of a multi-contact, tilt switch. This switch employs a mercury ball within a container that is proposed to be fixed in the pulse generator case, so that if the pulse generator is implanted at a certain orientation, and stays in that orientation, certain contacts are closed by the mercury ball when the patient is upright and others are closed or none are closed when the patient is prostrate, i.e., either prone or supine. During movement of the body, the mercury ball is expected to jiggle randomly and the number of contacts made per unit of time may be used as a measure of the level of activity. Similar sensors have been proposed in U.S. Pat. Nos. 4,869,251, 5,010,893, 5,031,618 and 5,233,984.
In the commonly assigned '984 patent, a cubic shaped multi-axis position and activity sensor is employed in rate responsive pacing applications and in the detection of tachycardia base on the patient being supine and inactive. In the commonly assigned '618 patent, a single axis position sensor is employed that is employed to control the therapy delivered by a spinal cord stimulator. The sensors in both patents employ conductive liquids, including an electrolyte or elemental mercury.
The use of elemental mercury is generally not favored and would increase environmental problems related to disposal of the pulse generators after use. Long term contact contamination and bridging issues would also arise, particularly given the extremely small size of the switch for confinement within modern pulse generator cases. To date, no implants of pacemaker pulse generators using such a tilt switch have been reported.
More recently, the use of a solid state position sensor in the form of a DC accelerometer is proposed in Alt U.S. Pat. No. 5,354,317. The DC accelerometer is fabricated in hybrid semiconductor IC form as a polycrystalline silicon, square plate, suspended at its four corners above a well in a single silicon crystal substrate, and associated low pass filter circuits are formed on the same substrate. The suspended plate structure moves between stationary positions with respect to the well on the suspension arms in response to earth gravity, depending on its orientation to the gravitational field. The plate also vibrates on the suspension arms similar to the AC accelerometer in response to acceleration movements of the patient's body.
In the pacemaker algorithms disclosed in the '317 patent, different base pacing rates are established depending on the static output of the position sensor that indicate the position of the patient, namely the upright, supine and prone positions, and separate base pacing rates can be set. Rate changes from the base pacing rates dependent on the exercise level of the patient in each position are suggested. Also, when changes in patient position are detected in the absence of physical exercise, the base pacing rate change is smoothed between the old and new rate to avoid a sudden step change.
The rate responsive pacemaker disclosed in the '317 patent offers some discrimination of patient position, but cannot distinguish among various patient positions where the suspended plate structure is aligned at the same angle to earth's gravitational field. The plane of the movable plate is at a fixed angle, e.g. coplanar, to a plane of the pulse generator case. Once the pulse generator is implanted in a patient, the movable plate plane may be aligned generally in parallel with the gravitational field and not detect the gravitational force (i.e., producing a zero amplitude output signal correlated to 0 g). The output of the so-aligned DC accelerometer would be the same whether a patient is standing, sitting or lying on either side, since the plate plane would remain in the same general parallel relationship to the gravitational field in all three positions. However, the pacing rates appropriate in standing, sitting or lying on a side are different when the patient is still.
The signal processing of the output signal from the single DC accelerometer of the '317 patent includes signal level calibration for each individual patient to account for differences in the angle of orientation of the DC accelerometer plate resulting from the implantation angle of the pulse generator case in the patient's body. However, this calibration is not suggested in order to distinguish body positions having a more or less common angular relation of the movable plate to the gravitational field.
Despite the weaknesses reported with respect to the piezoelectric sensors and solid state accelerometers, they remain favored over the other physiologic sensors that have been proposed or are in clinical use due to their relative simplicity, reliability, predictability, size, and low cost.